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Endemic- occurrence of expected number of cases among a group of
people over time
Incidence- # of new cases in a population
Prevalence- # of cases in a population (per 10,000 or 100,000)
Outbreak- more cases of a particular disease than expected in a given
area over a given time
Epidemic- large numbers of people over geographic area distribution
affected with the same disease
Pandemic- an epidemic spanning a very wide area
Vector- an animal intermediate that transmits a pathogen to humans
Virulence- Degree or intensity of pathogenicity of an organism
Compromised host- host with lowered resistance to infection
Nosocomial infection- an infection that is traced back to a hospital
Infectivity - capacity to cause infection in a susceptible host
Pathogenicity - capacity to cause disease in a host
Virulence - severity of disease that the agent causes to host
Case definition- The onset of ____ (symptoms) in a _____ (person) at
____ (time and place)
Confirmed- diagnosis by lab verification
Probable- many factors point to diagnosis, but no lab verification
Suspected- some factors point to diagnosis
Reservoir- site that harbors pathogenic organisms (human, animal, soil)
Morbidity rate- # sick divided by # exposed
Mortality rate- # dead per 100000 population
Case Fatality rate- # dead divided by # sick
Modes of transmission: droplet (through air, flu, TB, SARS,
hantavirus), blood (sexual or injected, HIV, hepatitis), direct contact
(touching, leprosy, chicken pox), oral-fecal (contaminated water,
cholera, giardia), vector (spread by animal, malaria, lyme disease)
AIDS- acquired immunodeficiency syndrome, spread by blood/
sexually, attacks immune system
Tuberculosis- caused by bacteria, cough, fever, fatigue, weight loss,
treated by antibiotics, attacks respiratory system or other parts of body
Malaria- caused by protozoan, spread by mosquitoes (anopheles),
cyclic fever and chills
2 Triads: Person, Place, Time; Agent, Host, Environment
Index Case: The first case in an outbreak
Virus: Viruses are small, much smaller than bacteria. They are not
composed of cells. Viruses have 2 basic components: DNA or RNA
covered in protein. Viruses can only reproduce inside the cells of
other living organisms (rabies, AIDS, SARS, ebola, measles)
Bacteria: Bacteria have 1 cell and no nucleus. DNA and ribosomes
float in the cell. They have flagella to help them swim. They have no
cell organelles. Gram + bacteria have a strong cell wall with
peptidoglycan and a capsule. Bacteria also have pili that help stick. (E.
coli, streptococcus, diptheria, MRSA, lyme disease)
Shapes: spherical (cocci)
Arrangements: staph (clumps)
Rod (bacilli)
Strep (chain)
Spiral (spirilla or spirochete)
Immunity Inherited-develops before birth, inborn
Acquired-Active/natural-exposed to antigen naturally
 Passive/natural-milk, placenta
 Active/artificial-injections, vaccines of antigens
 Passive/artificial-injections of antibodies
Lines of defense
1. Skin and secretions- acts as initial barrier, mucus catches pathogens,
enzymes kill pathogens
2. Inflammatory response- injury/tissue damage releases chemical signal,
blood flow increases: heat, redness, pain, swelling
3. Phagocytosis- ingests and destroys microorganisms: neutrophils,
macrophages
4. Natural killer cells- kills tumor cells and infected cells with viruses
5. Interferon- infected cell makes protein and releases into bloodstream,
interferes with reproduction
Epidemiology
 Study of health of population
 Uses scientific method
 Studies distribution and causes of disease in human populations
 Attempts to control these diseases investigates health concerns in
relation to disease
1)
Prepare for field work- Research disease, prepare to travel, make
arrangements with personal contacts
2) Establish the existence of an outbreak- compare current number of
cases to previous cases, use health records, documents, etc.
3) Verify diagnosis- Review clinical and laboratory results for the cases,
interview patients
4) Define and identify cases- establish case definition, have clinical info,
characteristics of the people, place, time, etc.
5) Describe and orient the data in terms of person, place, and time- use
epi curve to describe how many cases at what time
6) Develop hypotheses- consider disease, interview people who are ill,
try and notice what certain characteristics make people have the
disease
7) Evaluate hypotheses- compare with established fact, use statistics, use
case-control or cohort studies
8) Refine Hypotheses- study environment, use data for more insight
9) Control and Prevention measures- immunization, medicine, isolation,
carry out as soon as possible
10) Communicate findings- Oral briefing for local health authorities,
written report for archives
____________________________________________________________
Cohort Study- used for outbreaks in small, well-defined populations, moves
forward or backward from exposure
Disease?
Yes No
Exposed
(A) (B)
Unexposed (C)
(D)
Attack Rate- exposed A/(A+B)
unexposed C/(C+D)
Relative Risk- [A/(A+B)]/[C/(C+D)]
Relative Risk> 1: more likely
Relative Risk<1: possible protective effect
0-----------------------1------------------------
Possible protective effect
More likely
Case control Study- used when groups are not well-defined compares
people with the disease to people without, works backward
Exposed
Case
Controls
↓
Patients
Yes
(A)
(B)
No
(C)
(D)
Odds ratio: (A x D)/(B x C)
A= number of case patients exposed
B= number of control people exposed
C= number of case patients unexposed
D= number of control people unexposed
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Cholera- Vibrio Cholerae (oral-fecal)
Campylobacter Enteritis- campylobacter jejuni (oral- fecal)
Chicken Pox- varicella zoster (droplet and direct contact)
Chlamydia- Chlamydia trachomatis (sexually)
E. coli- Escherichia coli (oral-fecal)
Malaria-plasmodium (vector, anopheles mosquito)
MRSA- staphylococcus aureus (direct contact)
SARS-coronavirus (droplet)
Leprosy-mycobacterium leprae (direct contact)
Schistosomiasis- schistosoma (oral/contact with water)
Shingles-herpes zoster (contact, droplet)
Strep throat-streptococcus(droplet)
Tuberculosis- mycobacterium tuberculosis (droplet)
Tetanus-clostridium tetani (contact)
Ebola-filoviridae (contact/blood)
Athlete’s foot- tinea pedis (contact)
Jakob- Cruztfelt- prion(ingestion)
Tapeworm- nematode (ingestion)
Hepatitis- hepatitis a, b, c virus (a: oral fecal, b: sexually)
Giardia- giardia lamblia (direct contact)
Study design
Case-control
Cohort
Crosssectional
Experimental
or
Trial
Strength
Good for rare disease or
long latency, examine
multiple exposures from a
single outcome; less
expensive and quicker to
conduct than cohort study
Examining multiple
outcomes for a single
exposure; examine rare
exposures (such as asbestos
but not for rare disease); can
calculate the incidence of
disease (while case control
cannot); best technique for
an outbreak in a small, well
defined population; most
accurate observational study
Relatively short duration;
can study several outcomes;
least expensive
Most scientifically sound;
best measure of exposure
Weakness
Possible error in
recalling past exposure
(Recall Bias). Possible
time-order confusion
Not good for rare
diseases; costly in time
and resources; possible
loss to follow up over
time; factor, which may
be many years in the past
or may be seen as
socially (un)desirable
Since exposure and
disease status are
measured at the same
point in time, it may
not always be possible to
distinguish whether the
exposure preceded or
followed the disease.
Time consuming and
Expensive; Unethical for
Harmful Exposures
Hill’s criteria
Types of epidemic
 Point source - An epidemic in which all cases are infected
at the same time, usually from a single source or exposure.
 Continuous source - An epidemic in which the causal
agent (e.g. polluted drinking water, spoiled food) is
infecting people who come into contact with it, over an
extended period of time.
 Person-to-Person (a.k.a. Propagated) - An epidemic in
which the causal agent is transmitted from person to
person, allowing the epidemic to propagate
Path of infection
Reservoir:
Susceptible Host:
Portal of Entry:
Portal of exit:
Koch’s postulates
1)
2)
3)
4)
5)
6)
Collect samples from different people
Grow contents on Petri dishes
Look for similar organisms from each of the patients
Inoculate suspect organism into healthy animal
Wait for symptoms to occur
Isolate organism from diseased animals
1. Strength of Association - relationship is clear and
risk
estimate is high
2. Consistency - observation of association must be repeatable in
different populations at different times
3. Specificity - a single cause produces a specific effect
4. Alternative Explanations - consideration of multiple
hypotheses before making conclusions about whether an
association is causal or not
5. Temporality - cause/exposure must precede the effect/outcome
6. Dose-Response Relationship - an increasing amount of
exposure increases the risk
7. Biological Plausibility - the association agrees with currently
accepted understanding of biological and pathological processes
8. Experimental Evidence - the condition can be altered, either
prevented or accelerated, by an appropriate experimental process
9. Coherence - the association should be compatible with
existing theory and knowledge, including knowledge of past
cases and epidemiological studies